We are currently characterizing the abnormalities for regulation of human muscle glycogen synthesis in insulin-resistant subjects. In insulin-resistant subjects, fasting glycogen synthase phosphatase and phosphorylase phosphatase activities are reduced and fail to show the peak insulin stimulation observed for insulin-sensitive subjects at 10-20 minutes. Using specific inhibitors the abnormal enzyme activities were identified as a type-1 phosphatase (PP-1) in human muscle from insulin- resistant subjects. The reduced PP-1 activity observed in both glycogen and cytosolic subcellular fractions suggested an alteration in the catalytic subunit independent of the glycogen-bound regulatory subunit or G component. The abnormally low fasting PP-1 activity in insulin-resistant subjects persisted following trypsin treatment, suggesting that inhibitors 1 and 2 (characterized regulators of PP-1) are not important determinants of the abnormal phosphatase activity. Western blots indicate an increased concentration of catalytic subunit for PP-1 in the muscle from insulin-resistant subjects. This result suggests that the intrinsic activity or regulation of the catalytic subunit is abnormal in insulin-resistant subjects. The apparent affinity for cAMP activation of muscle cAMP-dependent protein kinase (PKA) is not decreased in insulin-resistant subjects following insulin infusion. In contrast, sensitive subjects show a reduced apparent affinity of their PKA regulatory subunit for cAMP following insulin infusion. This results in inactivation of the kinase and could stimulate glycogen synthase activity. These results suggest that abnormal regulation of both glycogen synthase phosphatase and PKA contribute to the insulin resistance which is secondary to abnormal insulin-stimulated glycogen synthase activity. Glucose metabolism also appears to be regulated in muscle by amylin effects on phosphorylase and abnormal insulin stimulation of glucose 1, 6-bisphosphate concentration in insulin resistant subjects.
